Dr. Henry Jekyll begins searching for the elixir of life, and discovers that female hormones prolong a person's existence. The potion he devises transforms him into a woman he calls Hyde, who kills prostitutes in order to provide material for his experiments. However, Hyde becomes increasingly dominant and Jekyll has to attempt to reverse the process...

For a film that purported to show for the first time a 'sex change in a film', Dr Jekyll and Sister Hyde (d. Roy Ward Baker, 1971) has a very interesting take on sexual politics. For example, Martine Beswick (who played Sister Hyde) allegedly said:

Sister Hyde is incredibly, blatantly sexy; loves men, revels in being a woman. That's how we all dream of being. Every woman's fantasy is to be a wife in the kitchen and a whore in the bedroom. Any girl who denies that is lying.

This famous quote comes from the film's press book, and was probably the work of a particularly imaginative copywriter. However, it is this suggestion of a certain Jekyll and Hyde tendency inherent in female nature that makes the premise of the film so appealing.

Beswick plays the part well, but interestingly was second choice for the role after Caroline Munro (who had appeared in The Abominable Dr Phibes (d. Robert Fuest, 1971) and later starred in Clemens' Captain Kronos Vampire Hunter (1974)) had turned down the role because of its nudity. In fact, there is just one nude scene in the film, immediately after Jekyll first turns into Hyde in the very clever transformation sequence. As the press book puts it:

A nude scene enacted by lovely screen actress Martine Beswick... has a very definite purpose... And the reason for the nudity? Why, surely a man who has suddenly changed into a staggeringly beautiful woman will want to inspect himself, and see what he looks like as a she!

As Jekyll, Ralph Bates is an ideal counterpoint to Beswick's cool, frosty demeanour, and the latent homosexual undertones that his foppish appearance suggest are allowed free reign when Hyde, trapped in Jekyll's body, tries to stroke Howard's face outside a lingerie shop. Bates met his future wife, Virginia Wetherell, on the production, in a scene in which Dr. Jekyll kills the prostitute she plays.

Sister Hyde is predicated on interesting ideas (not just the sex change, but the suggestion that Jack the Ripper may have been pursuing medical ends), which producer Brian Clemens allegedly jotted down on the back of an envelope before presenting it to executive producer Michael Carreras. It is also visually arresting; Clemens ensured that Beswick wore deep red to emphasise her bloodthirsty role, while toning down the red hues elsewhere.